DESCRIPTION: The Singer-Nicholson fluid mosaic model characterized the plasma membrane as a "two-dimensional oriented solution of integral proteins .....in the viscous phospholipid bilayer". Now, new information is forcing revision of some aspects of the model and has the potential to greatly enhance our understanding of membrane microstructure and its relation to cell function in normal and pathological situations. This information comes from attaching small particles to single or small groups of membrane components and following the motion of the particle with video microscopy or dragging the particle with the so called laser tweezers. Most proteins and even some lipids do not exhibit continuous, unrestricted lateral diffusion expected in a two-dimensional fluid. For example, many membrane components are transiently confined to small, submicron zones in seemingly undifferentiated regions of the plasma membrane. The basis of this proposal is to extend these particle tracking and manipulation experiments on several different fronts. In Specific Aim I, the physical characteristics and nature of zones confining membrane proteins and lipids to small domains is investigated. In Specific Aim II, experiments are proposed to assess whether this novel microstructure in the plane of the membrane is under explicit physiological control mediated in some cases by signal transduction pathways. In Specific Aim III, a new method of probing barriers to diffusion at multiple points on the cell simultaneously is proposed employing magnetic fields to move or "magnetophorese" particles attached to membrane components. Because this new information will alter the way we think about membrane structure and function, it is vital that we know all we can about the influence of the particle on these measurements; this is the goal of Specific Aim IV.